Piezoelectric devices, such as piezoelectric quartz filters, piezoelectric quartz resonators and the like, typically comprise a piece of piezoelectric material mounted to a substrate. In quartz devices, the quartz element of necessity has thin metallic electrodes attached to it through which electrical signals are coupled into and out of the piezoelectric quartz material. Common problems with piezoelectric devices are sufficiently isolating the piezoelectric devices from mechanical shock and dealing with mismatches in thermal expansion coefficients of the piezoelectric device and the substrate material.
Quite often, the components of the piezoelectric devices, such as a piezoelectric quartz material and the substrate, have different thermal expansion coefficients, this mismatch can cause mechanical stresses to be induced in the quartz during the life of these devices, as the quartz and substrate expand and contract over temperature variations. Further, mechanical shock transferred to the quartz through its mounting structure, can increase mechanical stresses that in addition to the thermal stress adversely affect the frequency and accuracy of these devices.
Various attempts over the years have been developed to mount piezoelectric quartz devices to a substrate. For purposes of this application, a compliant mount for a piezoelectric device, is a mounting device, apparatus or other mounting means that attempts to reduce or optimize mechanical stresses, on the piezoelectric quartz element.
Some prior art compliant mounting devices have used thin foil tabs that act as spring-type mounting structure that attempt to isolate the quartz element from its substrate. Other types of compliant mounting structures have attempted to use substrate materials having large thermal expansion coefficients which disadvantageously decouple the quartz from the base.
In FIG. 1, a prior art mechanical mount 10 is shown having foils 12 and epoxy bonds 14 and 16 coupling the quartz 18 and ceramic base 20. The inflection temperatures of this type of device approaches 25 degrees C. at best, and are difficult to manufacture in an automated fashion, because of such small dimensions and tight tolerances and the number of processes involved.
In FIG. 2, a second prior art rigid epoxy mount 30 is shown, which includes a rigid epoxy 32 coupling the quartz 34 and ceramic base 36 together. The inflection temperatures of this type of device are very high, such as about 30 degrees C. or higher. The quartz 34 is stressed during temperature variations, and mechanical shock resistance is minimal and frequency shifting due to these induced stresses is high.
A third prior art, silicone mount 50 is shown in FIG. 3, which includes a first conductive silicone 52 and a second layer of conductive silicone with a wraparound structure 54 coupling the quartz blank 56 with the ceramic base 58. The inflection temperature of this device approaches 28 degrees C.
Most, if not all, of the prior art compliant mounting schemes are difficult to use because of the small physical dimensions that modern piezoelectric quartz elements have. Conventional crystal oscillators using bent foil tabs to support a quartz and which are adhesively coupled by epoxy, as shown in FIG. 1, and the other prior art devices in FIGS. 2 and 3, have had marginal results, frequency shifting, increased mechanical stresses, poor shock resistance, and large unwanted frequency shifts over the temperature ranges of interest.
There is a need for an improved piezoelectric device for selectively reducing the frequency-temperature shift of piezoelectric crystals, to (i) minimize the mechanical stresses induced due to the thermal expansion mismatches between the base and crystal at a certain temperature range; (ii) provide a mechanically sufficient coupling such that the device can withstand mechanical shock; (iii) minimize frequency shifts over the temperatures of interest; (iv) provide a method of crystal attachment which is adaptable to mass production; and (v) provide a device with an inflection temperature substantially in the middle of the temperature range of interest, which can result in using quartz blanks with lower tolerances.
Accordingly, a low cost, readily manufacturable, higher quality compliant mount for a piezoelectric device would be an improvement over the prior art. A method by which quartz devices can be easily and reliably attached to a substrate and which isolates the quartz element from mechanical stresses would also be an improvement over the art.
It is also considered an improvement over the art to be able to make a low cost, high quality and readily manufacturable piezoelectric device with controllable temperature-frequency shifts over a temperature range of interest.